Device for holding mass spectrometer on a preset mass peak



Oct. 7, 195-2 Filed Nov. 15, 1949 c. F. ROBINSON DEVICE FOR HOLDING MASS SPECTROMETER oN A PRESET mss PEAK 2 SHEETS-SHEET l Arme/Vix Oct.

Filed NOV. l5, 1949 7, 1952 c. F. ROBINSON DEVICE FOR HOLDING MAss sPEcTRoMETER oN A PREsET MAss PEAK 2 SHEETS-SHEET 2 MEA/V EAM P05/770A C0. C. C @MRW/EN 7' RESL VIA/G SLI T /ow .55AM/V INVENToR.- (HA/uf: /r' Rom/:0M

'linear enclosure, say a tube.

Patented Oct. 7, 1952 oFFicE DEVICE Fon HOLDING MAss sPEoTRoM- Erna ONA PaEsE'r MAss PEAK Charles F. Robinson, Pasadena, Calif., assigner tol Consolidated Engineering Corporation, Pasadena, Calif., aj corporation fof California Application November 15, 1949,Serial.No. 127,411

'propelling or accelerating electrodes into and through an analyzer chamber. The analyzer chamber frequently takes the form of a curvi- During passage through the chamber, the heterogeneous beam of ions is subjected to a transverse electrical or magnetic eld,or both, to separate it into separate diverging ion beams', each of which is composed of ions of a given mass-*to-charge ratio which differs from the mass-to-charge ratio of the ions forming the other beams. The diverging beams may be successively focussed through an exit `or resolving slit in a terminal electrode onto .a collector electrocle. Alternatively. a single one of ythe ion beams may be continuously focussedvon 'the resolving slit `to the exclusion of the otherv beams. In either event, the focussing of the ion beam or beams is achievedv by adjustment of the v1l claims. (ol. l25o-"4.1m

The current produced by ion discharge at the collector electrode is indicative of the amountof ions in the particular Abeam and hence, constitutes a measure of the partial Vpressureof the molecules (from which the ions were derived) being analyzed. y l

As indicated above, itrmay be expedient to focus the mass spectrometer on a single mass peak or step-wise on two or more mass peaks rather than sweeping the'divergingvl ion. beams successively across the resolving slit. Such a xed focus instrument is used forvcontinuous monitoring where a continuous concentration of a single constituent in a sample stream is sought. in leak detection where the presence of an added contaminant is detected as it appearsy "in the atmosphere surrounding a vessel under `investigation, and generally under any circumstances in which a single component or a pre-selected groupl in the sample 2 v flection of the subjectl beaml fromja, centered position will affect theA apparent intensity of the beam vand hence. result in erroneous analysis. Since the factors which govern the position of the beam are in a typical instrumentthe transverse magnetic field and the accelerating voltage, deviation of eitherof these variables from a preselectedxed value will result in displacement of the beam from a centered position.

It has been proposedto stabilize the accelerating voltage against deviations from a pre-selected value by means cfa voltage-.dropping network wherein a fraction of the accelerating voltage is compared with the voltage output of astandard cell. AnyH variation in proportionality' between the fraction and the "standard voltage may be balanced automatically by a servo-motor or other means acting on the voltage source supplying the accelerating electrodes. However, the accuracy of such a system depends upona resistance. divider network, the long-term stabilityof which is questionable. Moroven'the possibility of loss of focus due to temperature sensitivity of the magnetic field, variations in power supply to an electromagnet, or aging of a permanentmagnet is in noway accounted for in such a voltage-dropping network. A- y l I have developed a method for holding a given ion-beam centered on the resolving slit which automatically compensates for fluctuations. in the accelerating voltage and variations in the strength of the magnetic eld.- The invention contemplates in ai method ofA mass spectrometry which comprises ionizing a sampleto be analyzed, propelling the ions by means of a C. accelerating potential into an analyzer region,.subjecting the ions in the analyzer region to a transverse magnetic'iield toseparate theml into individual ion beams, focussing `one of said ion beams .through a resolving slit, discharging Vthe beam ing-.beam from thel center of the resolving y*slit Aand adjusting. the accelerating potentialto maintain said mean position centered in saidv resolving slit.

The invention also includes one form of apparatus. for carrying out v this method `although other means for carrying out the methodarepossible.v In its preferred' form,` .the invention comprises in a mass spectrometer having. ananalyzer aeiasaa is focussed on the collector electrode, the combination comprising means for impressing an A. C.

component on the accelerating electrod'esto'oscill late the ion beam transversely with respect to they resolving slit, and means forfvarying the `D. C.

voltage applied from said source to the acceleraty l ing electrodes when the A. C.signal dueto oscillationv of the ion beam has the same fundamental frequency as component.

The invention will be more clearly understood by reference tothe following detailed description taken in relation to the accompanying 'drawing in which f Fig. 1 is a diagram of a .mass spectrometer in accordance with the invention;

Figs. 2, 2A and 2B are charts'showingg'respectively, the results in terms of collectorelectrode output when a given Iion beam is oscillated about a` centered position on the resolving slit, when vthe given ion beam i's oscillated about a widely decentered positionland when'th'e given ion beam is oscillajted about a slightly decentered position 'with respect 'to 'the slit where the amplitude of voscillation in each case is appreciably greater `than the width 'of the resolving s lit; and

Figs. 3 and 3Av vare charts showing, respectively, the results in terms of collector electrode output when a given ion beam is oscillated about a centered'position on the resolving slit andwhen the givenion beamis oscillated about' a decentered position with respect tothe slitl where the amplitude of oscillation in each case is only slightly greater than the width ofthe resolving slit. l l

Thel massJ matically in Fig. -1 includes a pair of accelerating electrodes I0, II and a repeller electrode I2. Ions are formed by an electron beam vtraversing the space between the` repeller electrode and the first accelerating electrode I and are-expelled from this region through a slit IDA in the accelerating electrode-by application of a positive potential to the repeller electrode. The expelled ions are acceleratedv between thev electrodes"A I0 and I II and are discharged from a slit -I I-A in the second electrode by vmeans of a potential impressed between the two accelerating electrodes.

The repelling and accelerating .voltages are applied to the electrodes from a voltage supply circuit I4 including a D. C. .source I5 connected in series with a slidewire resistor I6. The resistor It is tapped by s'liderI'I. connected to a Vresistor I8. iOne end of theresistor I-8 is'connected to the positivelside of the DI C. source I5 and is grounded at adjacent the connected end. vA lead 22 connects'the repeller' electrode to resistor I8 on the positive side of ground 2liandleads 23, 24 connect the two accelerating electrodes to spaced points on the resistor i8 on the negative side of ground 20. This arrangement represents a'conventional voltage supply circuit fora fixed-focus 'mass spectrometer wherein any 'change in the voltage delivered from the ksource or applied across resistor .I'8 results in proportionate change in the voltages applied to the two acceleratingelectrodes.

The ions issuing from the second accelerat- 'spectrometer shown diagram:-

4 ing electrode II are caused to pursue 'a curved path as indicated by ion beam 26, the curved path being produced as the result of a transverse magnetic field established by magnet 2T. The magnet 21 may be an electro-magnet or a permanent magnet, as desired. A terminal electrode is placed in the path of the ion beam,

" 'say after it has travelled through a 180 arc, and

the A. C. beam modulating,-

is provided Awith a resolving slit 3| through which a single beam of given mass-to-charge ratio may be focussed. A collector electrode 32 is disposed behindthe terminal electrode and ions passing .through the resolving slit 3| impinge on the collector electrode and are discharged to produce a current proportional to the number of ions in the beam. The collector electrode is electrically connected to a D. C. amplier 34 which amplies the discharged signal and feeds it to a sensing means 36 which may be a recorder or other sensing device. Tov this point the operation of the mass'spectrometer of Fig. l is typical ofv so-called fixed-focus instruments.

In accordance with the present invention, a source 38 `of A. C. voltage is coupled through a switch 38A and a transformer 39 to the D. C. voltage supply circuit to superimpose on the D. C. voltage applied to the accelerating electrodes a comparatively small A. C. component of a ireq'uency w. An A. C. amplifier 46 is connected through a switch 42to the collector electrode 32 sothat the`A. C. and D. C. amplifiers can be alternately connected to receive the discharge current. developed at the collector electrode.

This represents preferred construction at the present state of the art. although it is perfectly possible to use a single amplifier for both the A. C. and D. C. signals or to have both amplifiers permanently connected to the ion collector, as will be evident to one skilled in this art.

Oscillation ofthe 'ion beam about a centered position and with an amplitude greater than the vwidth w of the resolving slit, shifts the highest intensity portion; of the beam out of position so 4back to the centered position, the discharge signal is again at maximum intensity. Thus, an A. C. component appears in the collector electrodev output when the beam is oscillated. The frequencies and phases of the various components into which the A. C; signal may be resolved depends upon the mean position of the beam, provided only that the amplitude of oscillation is greater than as'will appear hereinafter. Since oscillation of the beam reduces the average intensity on the collector electrode, it is preferred practice -to oscillate it only while the collector electrode; is connected to the A. C. amplifier 40 through switch 42. For this reason, switches 42 and 38A are ganged sothat an A. C. component is impressed on the accelerating electrodes from source 38 only Vwhen the A. C. amplifier is connected to receive the output of the collector electrode.

A reversible motor 44 is mechanically coupled to the slider Il of the voltage supply circuit so that operation of `the motor will adjust the position of slider AI'I with respect to the slidewire I6. One coil 44A of the motor is connected to the A. C. source 3B and the other coil 44B of the motor is gnnected to the output of the A. C. amplifier In using the. apparatus of Fig. 1, switch 42 is conveniently a timing switch which connects the collector electrode to the D. C. amplifier for a great majority of 'the time and to the A5 C. amplifier 40 for a few seconds out of every `10 or l'minutes.

Theioperation'of the lapparatus can' be best described with relation to the diagrams of Figs. 2 andB. Figs. 2, 2A and 2B are graphs of the current developed at the collector electrode by an ion beam oscillating across the resolving slit with an amplitude considerably greater `thanlthe Width w ofthe resolving slit, the several figures showing the mean position-of the beam'on the slit axis, to the side of the slit and in an intermediate position, respectively. Figs. 3 and 3A depict the current developed at the collector elecamata? trode by an. oscillating .beam centered on the -plier 34, .theionbeam is not oscillated andthe setting of:.tap llremains fixed. Periodically, fsay every .15 minutes; switch 42 is adjusted to lconnect the collectorelectrodeto the A. C. amplifier,

at which time any variation in accelerating 4voltage or magnetic eld strength is compensated by appropriate adjustment of the slider. I1 so that the preselected ion beam will remain centered on the resolving slit 3 l.

Assuming'that switch` 42 is connected to the A. C. amplifier 40 and that A. C. source 38 is connected in the voltage supply circuit through switch 38A, as shown in Fig. 1, and that the ion beam 26 is perfectly centered on the resolving slit 3l., the situation depicted in Fig. 2 will pertain when the amplitude of oscillation is appreciably greater than `w.' The collector electrode output is shown as .pips 46, 46A, 46B, etc., corresponding to the periods when the ion beam 26 vis passing through the resolving slit. The oscillating ion beam and resolving slit are shown beneath the' output curve of the collector electrode'. At the zero position of the beam, considering the beam oscillation responsive to the A. C. compon'ent impressed on the accelerating electrode, lit

is perfectly centered on the resolving slit and a signal is applied to the A. C. ampli'er and through the amplifier to coil 44B of motor-44. A quarter of a cycle later the beam 26 has moved to the Vleft of theslit and no signal'is applied to the amplifier. Aquarter of a cycle later-the beam returns to the center of the slit and the signal 46A is developed.. I'he oscillation of the beam with respect to the slit throughout two complete cycles is shown in the Fig. 2L The first Fourier. componentof the discharge signal isfs'ur pips developed for each cycle as reflected in the oscillation of the ion beam.Y Under these'conditions, motor 44 will not be operative since'the frequency of thesignal applied to coilMB is 44Afromsource38. Y

A similar'graph is shown in Fig. 2A wherein the mean-beam position lies outside the resolving slit` Thus, as shown at the extreme left Vof Fig. 2A, the zero position of the ion beam is to the left of the resolving slit 3l. Oscillation of the beam carries it into theresolving-slitibut*the mean position isso yfar off center that the b e'am does not sweep across the'slit. Each time the twice the frequency of the signal applied to coil beam sweepsinto the slit, a signal represented by the pips 50, l50A, etc., is developedat the co1- lector electrode, thel duration of each signal depending 'on the vtime the beam isfocussed on the slit.-

jthis'instance, the rst'Fourier component i'slseen to have afundamental frequency o. When this signal is applied from the A. C. amplifier to the 'motor coil 44B, being of the same frequency as the A. C. signal applied to coil 44A, the motor 'will be energized, moving slider 'I1 in the"appropriatedirection to return the beam to a mean centered position. If the beam is decentered-to the right ofthe resolving slit, a signal similar to that shown in the graph of Fig.l 2A, but ofv opposite phase,` is developed. When such-a signall is applied to the motor coil 44B, the' motor will be energized the opposite direction to accomplish the necessary adjustment of'sii'tlerll. i

I InFig. 2B, the mean position'of the oscillating iongbeam lies 'off center onthe exit slit. i. e., intermediate the positions shown'in Figs. 2 and 2A. Agaiirfaijsignal; represented by pips 52, 52A, etc. "is "developed at 'the collector electrode throughoutA veach period that the oscillating bearinfalls on the slit. Where the beam decenteringl is sufficiently small that the beam sweeps across the slit, the pattern of the discharge `signal differs from that developed when the'bean'f is decentered to such an extent that it sweeps 'only apart of the Vslit `(Fig. 2A). vHowever, it' will be observed that the rst Fourier component of the discharge current as per Fig. 2B has a fundamental frequency wand hence, this signal applied to the rmotor coil will operate the motor'in theksamemanner as the signal developed under the conditions shown in Fig. 2A.

The exact appearance of the current pips shown in the drawings will depend on the relative magnitude ofthe slit width, the beam Width, and the oscillation amplitude. 'As an example of the variability of such appearance, Figs. 3 and 3A: show a mode of operation in which the beam isna'rrowerthan the slit and the modulation amplitude `is `lust large enough to move the beam to the slit edge.` Comparison of Figs. 2 and 2B vvithFigs. 3 and 3A will make it evident that although @the pip appearance is very different in the'two cases, the A. VC. signals developed by va centered or decentered 'beam are essentially identical withthecorresponding cases of Fig. 2.

A` significant feature of the invention, therefore; is that no matter how small the focussing error `may be, the signal developed at the col- .lectorelectrocle byan oscillating ion beam will have atwleast a first Fourier component of a 'frequency identical to the frequency of oscilla- -tionof the beam so long as the amplitude of oscillation is at least equal. to the width of the resolving slit. Onr the other hand, when the beam. is centered, Vthe rst Fourier component ofthe signal developed by an oscillating beam "device can be adjuste'd'to sense on a strong peak.

Thus, if the particularpeakfof interest is extremely weak, the switch'AZ can be connected the :voltage supply'circuitin such a fashion that switchingifrom a' D. amplierto thefA. C. amplifier will at thesame time .alter the accelerating voltage to -focus a stronger mass peak on the resolving slit. This can be accomplished, for example, by means of a stepper switch (not shown) in the voltage supply circuit operated responsive to the position of switch 42.- Thus, Ait is not essential that ythe instrument "center itself on the particular peak beinganalyzed since any of the factors contributing todecentering of a given peak will normallycontributeto decen tering of any other given peak.

constancyfof ampnscauonin the A.l c.. ampu' iieris' also not aVv problem provided only thatthe gain remains large enough Vto drive the motor. The system balances toa null vwith respect to the fundamental component of the A. @ampliier output so that variation in gain-within the range sufficient yto operate themotor canr be tolerated. Furthermore,V motors can be` obtained commercially which automatically discriminate against noise and extraneous frequencies which, therefore, do not interfere with the operation of the apparatus so long as they are not present in suicient amounts to jam the motors by overdamping.

The method and apparatus is simpler, less expensive, and .more certain .than any means heretofore. proposed for holding a mass spectrom eter on the top Aof a pre-set 'peak or a group of peaks by attempting to vachieve a high degree of long-term stability in'the .magnetand inthe power supply.

Although the invention has been described with particular reference -to a continuous monitor instrument in which a single mass peak is of interest, it is not so limited. In any operation wherein the mass peaks are focussed step-wise on the resolving slit, the present invention is of value.

Furthermore, application of the invention is not limited to the particular apparatus illustrated. It is entirely practical to produce the necessary oscillation of the ion beam by means othei` than impression of an A. C. voltage on the accelerating electrodes. ForA example, auxiliary electrodes arranged transversely with respect to the path of ion travel will induce the same type of oscillation when energized with an A. C. voltage. Alternatively, the magnetic field may be varied in a rhythmic manner to accomplish the same purpose. At the same time, the correction applied by motor MI. or by other meansy sensitive tov ther Fourier component of thefcollector electrode output of the frequency of beam oscillation, is not restricted to adjustment of the accelerating potentials. Variation vin the magnetic field strength or an auxiliary eld developed by auxiliary deiiecting electrodes will also accomplish the desired adjustment of the beam.

The. method ofthe invention, therefore, involves loscillating the ion beam in any desired manner and adjusting a beam focussing field, whether it be a lateral or transverse electric eld or a transverse magnetic field, responsive to the Fourier component of collector electrode output having the same frequency as the oscillation frequency of the ion beam.

I claim:

1. In mass spectrometry involving ionizing a `sampleto be analyzed, propelling the ions by means of a D. C. accelerating potential into an jecting the ions in the analyzing zone to a transverse magnetic field to separate the heterogeneous beam into a plurality of diverging beams, focussing one of said beams on an outlet slit in the analyzing zone, discharging the ion beam passing ,through the outlet slit, and measuring the vcurrent developed by discharge of the beam, the improvement which comprises superimposing on the D. C. acceleratingpotential an A. C. component to cause the ion-beam to oscillate transversely with respect to the outlet slit and at an amplitude such that the beam will impinge on an edge of said outlet slit at least once in each cycle of oscillation, detecting the mean position ofthe oscillating beam, and adjusting the accelerating potential tomaintainl said mean position'centered on .said outlet slit.

2.. In mass spectrometry involving ionizing a sample to be analyzed, propelling theions by means of aA D. C. accelerating potential into an analyzing zone as a heterogeneous beam, sub- `iecting the ions in the analyzing zone to a transverse magnetic field to separate the heterogeneous beam into a plurality of diverging beams, focussing one of said beams on an outlet slit in the analyzing zone, discharging the ion beam passing through the outlet slit, thek improvement which comprises `periodically superimposing on the D. C. accelerating potential an A. C. component to cause the ion beamv to oscillate transversely with respect to the outlet slit and at an amplitude such that the beam will impinge on an edge of .said outlet slit at least once in each cycle of oscillation, detecting the mean position `of -the'oscillating beam, and adjusting the accelerating potential to maintain said mean positoncentered on said outlet slit.

3. In mass spectrometry involving ionizing a sample ,to be analyzed, propelling the ions by means of a D. C. accelerating potential into an analyzing zone.` as a heterogeneous beam, subjecting the ions in the analyzing zone to a transverse magnetic field to separate the heterogeneous beam into a plurality of diverging beams, focussing one of said beams on an outlet slit in vthe analyzing zone, discharging thev ion beam .meanposit-ionof the oscillatingbeam, and adjusting the accelerating' potential responsive to variations in the mean beam position to maintain lsaid mean position centered on said outilet slit.

4. In vmass spectrometry involving ionizing a sample to be analyzed, propelling the ions by meansof a D. C..accelerating potential into an `analyzing zone as a heterogeneous beam, subjectingi the ionsin the analyzing zone to a transverse magnetic fieldA to separate the heterogeneous beam into a plurality. of diverging beams, focussing one of said beams on an outlet slit in the analyzing zone, discharging the ion beam vpassing through the outlet slit, and measuring the -current developed by discharge of the beam, the e improvement which comprises periodically superimposingon the D. C. accelerating potentialan A.' C.l component to cause the ion beam to oscillate transversely with respect to the outlet slit so that the discharge signal of said beam aeiasez includesfan A. C.rcomponent,andadjustingthe y accelerating potential' responsive to the A. C. com-v ponent of said discharge signal when saidcomponent is of the samefrequency as the "A: C. com# ponent superimposedon the` accelerating poten,- tial toy maintain the mean position ofi.the=oscil lating beam centered ,on.zsaid outletslit.

5. In mass spectrometryinvolving ionizing. a sample to. be'. analyzed, :propelling the. ions.' by means of a D. C. accelerating potential intoan analyzing zone as aheterogeneous beam, subjecting the ionsinthe analyzing zone to a transverse 'magnetic field xto separate the heterogeneous beam into aplurality I'of'divergingv beams, focussing one :of said beams on an outlet slit in the analyzing zonefdischarging the ion beam passing throughthe outlet slit, and measuring the current developed yby discharge of the beam,

the improvement vvliiclrfcomprises periodically superimposing onthe'D. C. accelerating potential an A. C. component of suicient magnitude to cause the ion beamftopscillate transverselyA with respect to the outlet slit with an amplitude greater than the, widthof the ion slitso that sample to'be analyzed; propelling the ions 'by means of a D. C. accelerating potential into `an analyzing zone as a heterogeneous beam, subjecting the ions in the analyzing zone to a transverse magnetic field to separate the heterogeneous beam into a plurality of diverging beams, focussing one of said beams on an outlet slit in the analyzing zone, discharging the ion beam passing through the outlet slit, and measuring the current developed by discharge of the beam, the improvement which comprises periodically superimposing on the D. C. accelerating potential an A. C. component to cause the ion beam to oscillate transversely with respect to the outlet slit so that the discharge signal of said beam includes an A. C. component, and automatically adjusting the accelerating potential responsive to the A. C. component of said signal when said component is of the same frequency as the A. C. component superimposed on the accelerating po-` tential to maintain the mean position of the oscillating beam centered on the outlet slit.

'7. In mass spectrometry involving ionizing a sample to be analyzed, propelling the ions by means of aD, C. accelerating potential into an analyzing zone `as a, heterogeneous beam, subjecting the ionsin the analyzing zone to a transverse magnetic eld to separate the heterogeneous beam into a plurality of diverging beams, focussing one of said beams on an outlet slit in the analyzing zone, discharging the ion beam passing through the outlet slit, and sensing the current developed by discharge of the beam, the improvement Which comprises periodically altering the accelerating potential by a given increment to focus a different ion beam on the outlet slit, superimposing on the altered accelerating potential an A. C. component to cause said different beam to oscillate transversely with respect to the outletslit so that the discharge signal of said different beam includes an A. C. component, adjusting the altered accelerating potential'responsive to the/A. C.-component of said signal when said= component is of the'same frequency as said superimposed A.` C. component'to maintain the focus of said different ion beam on the center of the "outlet slit, and altering the adjusted accelerating voltageby saidy given increment to again focus said one of said ion beams on the outlet slit. S Y

- '8.' In a mass spectrometer having `an'analyzer chamber, ionization chamber, Aaccelerating electrodes,` a source of DAC. voltage connectedkto impress an acceleratingpotential on the accelerating electrodes for propelling ions from the ionization chamberinto and through the analyzer chamber, means for establishing a' transversemagnetic field in the analyzerchamber, a collector A"electrode, and a resolvingA electrode havingsa resolving Aslit through which an'ion beamfis "focussed `on the collector electrode, the combination comprising' means for impressing an A. C."voltage'component on the accelerating electrodes to oscillate the ion beam transversely with respect'to the resolvinglslit, means for developing an A.'"`C.signal at saidlcoll'ector electrode responsive'` to oscillation of the ion beam and means Vfor Vvarying the D. C. voltage applied from said source to the accelerating electrodes WhensaidA. C. signal vhas the same fundamental r'frequency as said A. C. component.

` 9'. In a mass spectrometer having ananalyzer chamber. ionization chamber, accelerating velectrodes, a source of D. C. voltage connected to impress an accelerating potential ori-the accelerating electrodes for propelling ions from the ionization chamber into andv through the' ana'- lyzerchamber, means for establishing a trans-i verse magnetic'iield in` the analyzer chamber, a collector electrode," andv a resolving electrode having a resolving slit through which ions pass to the collector electrode, the combination comprising means for impressing an A. C. voltage component on the accelerating electrodes, and means operable responsive to the output of the collector electrode having the same frequency as said A. C. component to vary the D. C. voltage applied from said source to the accelerating electrodes.

l 10. In a mass spectrometer having an analyzer chamber, ionization chamber, accelerating electrodes, a source of D. C. voltage connected to impress an accelerating potential on the accelerating electrodes for propelling ions from the ionization chamber into and throughthe analyzer chamber, means for establishing a transverse magnetic field in the analyzer chamber, a collector electrode, and a resolving electrode having a resolving slit through which ions pass .to the collector electrode, the combination 'comprising'means rfor impressing an A. C. voltage component on the accelerating electrodes, an A. C. amplifier connected to the collector electrode, means operable responsive to the output of the A. C. amplifier when said output is at the same frequency as said A. C. component to vary the D. C. voltage applied from said source to the accelerating electrodes.

il. In a mass spectrometer having an analyzer chamber, ionization chamber, accelerating elec trodes, a source of D. C. voltage connected to impress an accelerating potential on the accelerating electrodes for propelling ions from the ionization chamber into and through theanalyzer chamber, means for establishing a transverse magnetic eld in the analyzer chamber, a collector electrode, and a terminal electrode having a resolving slit through which ions pass to the collector electrode, the combination comprising a source of A. C. voltage connected to impress an A. C. voltage component onthezaccelerating electrodes,:an A. C. amplifier connected to the collector electrode, adjusting means :for varying the D.v C. -voltage applied from 'said source to the accelerating electrodes, al motor mechanically connected to control said adjusting means, one coil of said motor being connected to said source of A. C. voltage andanother coil of themotor being connected to the output of said A. C. ampli-fier so that the motor is actuated only when the output of said amplifier vhas a component of the same frequency as the A. C.

voltage.

12. In amass spectrometer having :rn-analyzer chamber, ionization chamber, Iaccelerating electrodes, a D. C. voltage supply circuit connected vto `impress-an accelerating potential on -theaccelerating electrodes for propelling ions 'from ythe ionization-chamber into and through the-analyzer chamber, -means for establishing `a transverse magnetic'eld in the analyzer chamber, a collector electrode, and a terminal electrode having a resolving slit through which ions pass to the co1- lector electrode, the combination comprising a source of A. C. voltage connected t0 said D. C. voltage supply circuit to impress an A. C. voltage component on the accelerator electrodes, an AJC.

amplifier connected to the collector electrode, a rst switch means for disconnecting the source of A. C. voltage from said supply circuit, second switch means for disconnecting the A. C. amplier from the collector electrode, means for simultaneously operating the iirst and second switch means, adjusting means for varying the voltage applied from said supply circuit to the accelerating electrodes, a motor mechanically connected tocontrol said adjusting means, one coil of the motor .being connected to the v:source of A. C. voltage and another coil of the motor being connected to lsaid A. C. amplier so that the motor is actuated 'only when .the output of said rampliiier has'fa component of the same frequency as the A. C."voltage.

13. In mass spectrometry involving ionizing a sample to be analyzed, subjecting the ions to a combination of electric and magnetic fields to separate the ion beam into a plurality of divergent beams each-having a characteristic massto-charge ratio, and passing one such divergent beam through va resolving slity onto a collector, the improvement which comprises causing one of said Vbeams ito oscillate across the resolving slit and lsensing onthe Fourier component of the collector current signalwhose frequency is the same as the frequency with which the beam is oscilla-ted across the slit, and adjusting the position of the .beam with respect to the slit responsive to said Fourier component.

'14. vIn'ama'ss spectrometer having an analyzer chamber, ionizationchamber, accelerating electrodes for propelling ions from the ionization chamber through the `analyzer chamber, means for establishing a transverse electric iield across the analyzer chamber, a collector electrode and aterminalelectrode preceding the collector electrode and having a resolving lslit therein, the combination comprising means for oscillating an ion beam with respect to said resolving slit, and means associated with the collector electrode and operable responsive to uncentered oscillation oi the beam to vary the beam focus to restore centered oscillation of the beam.

Y CHARLES F. ROBINSON.

No references cited. 

